Gamma ray collimating device



Jan. 27, 1959 L. S. GOURNAY cum RAY COLLDIATING bsvrcs Filed Aug. 1, 1955 f i 7 I L "i F IG. 3.

FIG. 4.

LUKE S. GOURNAY INVENTOR.

A TTUR'NE Y United States Patent GAMMA RAY COLLIMATING DEVICE Luke S. Gournay, Oxnard, Califl, assignor, by mes ne to Socony Mobil Oil Company, Inc., New

assignments, York, N. Y., a corporation of'New York This invention relates to the collimation of rays emitted by radioactive substances and relates more particularly to a device for the collimation of gamma rays.

In the study of radioactive substances, it is frequently necessary that selected segments of the radioactive source be studied separately. To make such studies it is required that the segments to be studied be isolated so that they may be examined without interference from other segments of the same radioactive source. Isolation of the selected segments may be effected by collimation of the rays emitted by the segments in question or, in other words, by the shielding of. all rays emitted by the source except those rays which. are traveling in essentially the same direction from the segment in question.

Collimation of rays from radioactive sources has been accomplished in the past in numerous different ways. Some collimation devices have utilized absorbing or shielding materials such as lead, iron and tungsten alloys. One disadvantage of lead shielded collimation devices is the background which exists due to the inherent radio activity of the lead. Other collimation devices have utilized Geiger-Muller counters connected in coincidence circuits.

Counters in coincidence circuits are connected together to form a counting system which will register acount only when a single ray penetrates each counter. substantially simultaneously or each counter of thesystem is penetrated by separate rays simultaneously. It will be appreciated that in a coincidence system wherein a plurality of counters are arranged with their longitudinal axes parallel and in a common plane there is a substantial lessening of the possibilities that a single ray will penetrate each of the counters substantially simultaneously or that each of the counters in the system will be penetrated by separate random rays simultaneously. As a practical matter it has been found that in a coincidence system as just discussed, the counting rate for the system is reduced approximately one hundred fold for each counter which is added to the system.

It is an object of this invention to collimate rays emitted by a radioactive source. It is another object of this invention to provide apparatus which will permit a small segment of a large radioactive source to be examined independently of other segments of the same source. It is another object of this invention to provide apparatus for the collimation of high energy gamma radiation such that the gamma rays reaching a radiation detector are all traveling in essentially the same direction. Other objects of the invention will become apparent from the following detailed description.

In accordance with the invention, there is provided an apparatus containing an air-filled slit which forms an exclusive path for gamma rays from a radioactive source through a body of mercury to one or more radiation detectors. The radiation detectors are contained within an enclosure which, like the air-filled slit, is surrounded by the body of mercury.

In the drawings Figure 1 is a view in perspective of a preferred form of collimating device constructed in accordance with my invention. I

Figure 2 is a top view of the device of Figure 1.

Figure 3 is a front view of the device of Figure 1.

Figure 4 is a side view of the device of Figure 1.

It has been found that mercury is superior to those materials previously used for shielding rays from radioactive substances in collimation devices. Mercury shielding provides greater efliciency in collimation than lead shielding without a sacrifice in intensity of gamma radiation at the point where the gamma rays are counted. A lead shielded device providing the same degree of shielding as a mercury shielded device requires a longer collimation slit and more lead since lead does not absorb the radiation as readily as mercury. Use of a longer collimation slit permits more rays to scatter and reflect resulting in fewer rays actually reaching the radiation detectors. In addition, a system using mercury as the shielding material has less background radiation than systems using other decay elements for shielding due to the fact that the inherent radioactivity of mercury is less than that of other decay elements.

Referring to the drawings, box 5 comprises top 6, bottom 7, back 8, front 9, and sides 10 and 15. Box 5 is formed of a structurally rigid material which is transparent to gamma rays and chemically inert to mercury. With the use of mercury as a shielding medium, the ma-' terial may be a polymethyl methacrylate such as is sold under the trademark Lucite. This material is transparent to light rays, and the box as illustrated in the drawing is constructed of transparent materials. How ever, such transparency is not required and a material opaque to such rays may be used. Valve 20' is posi tioned in top 6 to permit the introduction of mercury into the box. Valve 21 is positioned in side 10 near bottom 7 to provide means for withdrawing mercury from the box. The positions of valves 20 and 21 in box 5 may be varied as desired so long as means are provided fbor mercury to be introduced into and withdrawn from the Tube assembly 22 as shown comprises tubes 23 and 24 which are joined together with their longitudinal axes parallel to each other. Tubes 23 and 24 are joined together in a plane which is perpendicular to a plane pass,-

, ing through the longitudinal axes of the tubes and is parallel to the longitudinal axes of the tubes. One end of the tube assembly is joined to side 15 such that said end is closed or sealed by the inside plane surface of side 15. The other end of the tube assembly is secured to side 10 and passes through and terminates flush with the outside plane surface of side 10. Thus, tube assembly 22 is sealed from the'inside of box 5 and is accessible from the outside of the box through the end of the tube assembly terminating in side 10'. If desired, the end of the tube assembly terminating at side 15 may extend through side 15 as does the end of the assembly terminating in side 10, thus permitting radiation detectors to be inserted through either end of the tube assembly. The number of tubes in the tube assembly may be varied as desired depending upon how many radiation detectors are used as counters. The number of detectors to be used will afie ct the countingrate. As previously pointed out, the addition of each detector in a coincidence circuit system reduces the counting rate for the system. Each tube in the assembly will accommodate one radiation detector, such as a Geiger-Muller tube. The tube assembly is constructed of a material which is transparent to gamma rays and chemically inert to the'liquid shield ing medium used. It may be formed of the same material which is used in constructing box 5.

Front side 25 of tube assembly 22, a flat surface formed on tube 24, is parallel to the longitudinal axes of the tubes and is perpendicular to a plane passing through the ,longitudinal axes of the tubes. Air-filled slit assembly 30 is supported within box 5 between side of tube assembly 22 and the inside plane surface of front 9. Slit assembly is provided with top 31, bottom 32 and the sides 33 and 34. The internal plane surfaces of top 31, bottom 32, and the sides 33 and 34 form the boundaries of a slit 35 passing through the slit assembly. Top 31 and bottom 32 are parallel to and equidistant from a plane passing through the longitudinal axes of tubes 23 and 24. In the embodiment of the invention illustrated the slit assembly is constructed of stainless steel sheet material, although it will be recognized that it may be constructed of any structurally rigid material which will retain its shape when used in relatively thin sheet form and will not be afiected by the mercury surrounding the slit assembly. It is desirable that the maximum amount of radiation reaching the detectors from the segment of the sample pass through slit 35 rather than through the material of which the slit assembly itself is constructed. The material used to form the slit assembly will probably have a linear coefficient of absorption much less than that of mercury. Accordingly, it is important that the thickness of the material be no more than required for physical rigidity of the assembly.

The length of the slit assembly, which for purposes of this description will be considered as the distance from front side 25 of tube assembly 22 to the inside plane surface of front 9, may be varied in accordance with the degree of collimation desired. Increasing the length of the slit assembly will increase the collimation obtained, while decreasing the length of the slit assembly will decrease the degree of collimation obtained. The width of the slit assembly, which is its dimension taken along the length of the tube assembly, may be varied as desired in accordance with the active length of the detectors used. The thickness of the slit assembly, i. e., that dimension of the assembly taken along a line parallel to side 25 and perpendicular to a plane passing through the longitudinal axes of tubes 23 and 24, may be varied as desired depending on the thickness of the segment of the radioactive source to be examined. One open end of the slit assembly bears tightly against the inside plane surface of front 9, while the other open end bears tightly against front side 25 on tube 24. The end of the slit assembly terminating at front 9 may, if desired, pass through the front and terminate flush with the outside plane surface of the front.

With tube assembly 22 and slit assembly 36 securely mounted within box 5, the box is filled through valve 20 with mercury. Valve 21 is closed during the filling process. The mercury surrounds but does not enter the tube and slit assemblies. The detectors are inserted into the tube assembly and the segment of the radioactive source to be studied is placed in front of the slit assembly adjacent to front 9. Those rays emitted by the segment of the radioactive source which are traveling in essentially the same direction and approximately parallel with the slit pass through the slit to the detectors where they are counted. The rays emitted by the other segments of the radioactive source and the rays from the segment being studied which are not traveling in a direction essentially parallel to the slit all pass into the mercury where they are absorbed. Thus, only a selected group of rays from the segment being studied are permitted to reach the detectors for counting.

4; cover such modifications as fall within the scope of the appended claims.

I claim:

1. In an apparatus for the collimation of gamma rays the combination of a substantially enclosed box, a body of mercury contained within said box substantially filling said box, a valve positioned in the top of said box to permit'the introduction of said mercury into said box, a valve positioned in said box near the bottom thereof to permit the withdrawal of said mercury from said box, a tube assembly comprising at least one hollow cylindrical tube secured within said box approximately midway between the top and the bottom of said box and having at least one end opening through a side of said box to permit the introduction of gamma ray detecting means into said tube assembly, said tube assembly being sealed with the inside surfaces of said box to exclude said mercury from said assembly, a slit assembly secured between said tube assembly and the inside surface of the front of said box, said slit assembly being sealed to exclude said mercury to provide an exclusive air-filled path for gamma rays from the front of said box through said mercury to said tube assembly.

2. In an apparatus for the collimation of gamma rays the combination of an enclosed box formed of a material transparent to gamma rays and chemically inert to mercury, a body of mercury contained within and substantially filling said box, a valve secured in the top of said While the invention has been described in connection I with certain specific embodiments thereof, it will now be understood that further modifications will suggest themselves to those skilled in the art and it is intended to box to permit the introduction of said mercury into said box, a valve secured in said box near the bottom thereof to permit the withdrawal of said mercury from said box, a tube assembly comprising at least one hollow cylindrical tube formed of a material transparent to gamma rays and chemically'inert to mercury secured in said box between the sides thereof approximately midway between the top and bottom of said box, one end of said tube assembly passing through and sealed with one side of said box and the other end of said tube assembly being sealed to the inside plane surface of the other side of said box, the front side of said tube assembly having a plane surface formed thereon, and an air-filled slit assembly having one end secured and sealed to the front plane surface of said tube assembly and the other end secured and sealed to the inside plane surface of the front side of said box.

3. In an apparatus for the collimation of gamma rays the combination of an enclosed box formed of a material transparent to gamma rays and chemically inert to mercury, a body of mercury contained within and substantially filling said box, a valve secured in the top of said box to permit the introduction of said mercury into said box, a valve secured in said box near the bottom thereof to permit the withdrawal of said mercury from said box, a tube assembly comprising two hollow cylindrical tubes formed of a material transparent to gamma rays and chemically inert to mercury secured in said box between the sides thereof approximately midway between the top and bottom of said box, one end of said tube assembly passing through and sealed with one side of said box and the other end of said tube assembly being sealed to the inside plane surface of the other side of said box, the front side of said tube assembly having a plane surface formed thereon, and an air-filled slit assembly having one end secured and sealed to the front plane surface of said tube assembly and the other end secured and sealed to the inside plane surface of the front side of said box. 7

4. In an apparatus for the collimation of gamma rays the combination of an enclosed box formed of a material transparent to gamma rays and chemically inert to mercury, a body of mercury contained within and sub stantially filling said box, a valve secured in the top of said box to permit the introduction of said mercury into said box, a valve secured in a side of said box near the bottom thereof to permit the withdrawal of said mercury from said box, a tube assembly adapted to receive a detector for gamma rays and comprising at least one hollow cylindrical tube formed of a material transparent to gamma rays and chemically inert to mercury, said tube assembly being secured in said box between two sides thereof substantially midway between and parallel to the bottom and top of said box, one end of said tube assembly opening through and sealed around its periphery with one side of said box and the other end of said tube assembly terminating in sealed relationship with the inside plane surface of the other side of said box, said tube assembly having formed on the front thereof a plane surface parallel to the inside plane surface of the front of said box, and an air-filled slit assembly secured between said plane surface on said tube assembly and said inside plane surface of the front side of said box to provide an exclusive defined path for gamma rays from the front side of said box through said mercury to said tube assembly.

5. In an apparatus for the collimation of gamma rays the combination of an enclosed box formed of a material transparent to gamma rays and chemically inert to mercury, a body of mercury contained within and substantially filling said box, a valve secured in the top of said box to permit the introduction of said mercury into said box, a valve secured in a side of said box near the bottom thereof to permit the withdrawal of said mercury from said box, a tube assembly adapted to receive gamma ray detector means and secured between two sides of said box substantially parallel to and midway between the top and bottom of said box, said tube assembly comprising two hollow cylindrical tubes joined together in a plane parallel to the longitudinal axes of said tubes and perpendicular to a plane passing through the longitudinal axes of said tubes, the front side of said tube assembly being provided with a plane surface formed parallel to the longitudinal axes of said tubes and perpendicular to a plane passing through the longitudinal axes of said tubes, one end of said tube assembly being in sealed relationship with the inside plane surface of one side of said box, the other end of said tube assembly opening through and sealed around its periphery with the other side of said box to permit the introduction of gamma ray detector means into said assembly, and an air-filled stainless steel slit assembly secured at one end in sealed relationship to said plane surface on the front side of said tube assembly and secured at the other end in sealed relationship to the inside plane surface of the front of said box, said slit assembly being rectangular in cross section in a 'plane parallel to the plane surface on the front of said tube assembly, and said slit assembly being adapted to provide an exclusive path for gamma rays from the front side of said box through said body of mercury to said tube assembly.

Pajes June 2, 1953 Stewart et al Apr. 13, 1954 

